The present invention relates to phosphor compositions, particularly phosphor compositions for use in fluorescent lamps. More particularly, the present invention relates to improving the CRI of a fluorescent lamp by providing an optimized phosphor blend including a limited amount, up to only 20% by weight of the phosphor, of rare earth phosphor in combination with other broad band phosphors for use therein.
Fluorescent lamps typically have a transparent glass envelope enclosing a sealed discharge space containing an inert gas and mercury vapor. When subjected to a current provided by electrodes, the mercury ionizes to produce radiation having primary wavelengths of 185 nm and 254 nm. This ultraviolet radiation, in turn, excites phosphors on the inside surface of the envelope to produce visible light which is emitted through the glass.
Generally, a fluorescent lamp for illumination uses a phosphor which absorbs the 254 nm Hg-resonance wave and is activated to convert the ultraviolet luminescence of mercury vapor into visible light. Conventionally, a white-emitting calcium halophosphate phosphor, such as Ca10(PO4)6(F,Cl)2:Sb,Mn, has been used to convert the UV light to white light. More recently, in order to improve the color-rendering properties and emission output of fluorescent lamps, a three-band type fluorescent lamp, which employs a mixture of red, green, and blue-emitting phosphors, has been used to render illumination of a white color. For example, the phosphor may include a mixture of europium-activated barium magnesium aluminate phosphor (BaMg2Al16O27:Eu2+) for the blue-emitting phosphor, cerium and terbium-activated magnesium aluminate phosphor (Ce, Tb)MgAl11O19 for the green-emitting phosphor, and europium-activated yttrium oxide phosphor (Y2O3:Eu3+) for the red-emitting phosphor, mixed in an adequate ratio. The combined spectral output of such a phosphor blend produces a white light.
The apparent, or perceived, color of a light source is described in terms of color temperature which is the temperature of a black body that emits radiation of about the same chromaticity as the radiation considered. A light source having a color temperature of 3000 Kelvin, i.e. 3000K, has a larger red component than a light source having a color temperature of 4100K. The color temperature of a lamp using a phosphor blend can be varied by changing the ratio and composition of the phosphors.
Color quality is further described in terms of color rendering, and more particularly color rendering index (CRI or Ra), which is a measure of the degree to which the psycho-physical colors of objects illuminated by a light source conform to those of a reference illuminant for specified conditions. CRI is in effect a measure of how well the spectral distribution of a light source compares with that of an incandescent (blackbody) source, which has a Planckian distribution between the infrared (over 700 nm) and the ultraviolet (under 400 nm), i.e. in the visible portion of the spectrum. The discrete spectra which characterize phosphor blends will yield good color rendering of objects whose colors match the spectral peaks, but not as good for objects whose colors lie between the spectral peaks. Lamp CRI can be improved by using an appropriate combination of rare earth phosphors or by using phosphors emitting broadband spectral distribution.
The color appearance of a lamp is described by its chromaticity coordinates which can be calculated from the spectral power distribution according to standard methods. See CIE, Method of measuring and specifying color rendering properties of light sources (2nd ed.), Publ. CIE No. 13.2 (TC-3, 2), Bureau Central de la CIE, Paris, 1974. The CIE standard chromaticity diagram includes the color points of black body radiators at various temperatures. The locus of black body chromaticities on the x,y-diagram is known as the Planckian locus. Any light emitting source represented by a point on this locus may be specified by a color temperature. A point near but not on this Planckian locus has a correlated color temperature (CCT) because lines can be drawn from such points to intersect the Planckian locus at this color temperature such that all points on a given line look to the average human eye as having nearly the same color. Luminous efficacy of a source of light is the quotient of the total luminous flux emitted by the total lamp power input as expressed in lumens per watt (LPW or lm/W).
Spectral blending studies have shown that the LPW and CRI of white light sources are dependent upon the spectral distribution of the individual color phosphors. It is expected that such phosphors preserve structural integrity during extended lamp operation such that the phosphors remain chemically stable over a period of time while maintaining stable CIE color coordinates of the lamp. The human eye does not have the same sensitivity to all visible light wavelengths. Rather, light with the same intensity but different wavelengths will be perceived as having different luminosity. The use of tri-phosphor blends has led to improvements in color rendering as compared to single phosphor lamps.
For example, US Pub. App. 2007/0170834, published on Jul. 26, 2007 and sharing a common inventor herewith, discloses a phosphor blend including a strontium rare earth phosphor (Sr4Al14O25: Eu2+) that achieves a CRI of 90+, at a low CCT. However, the rare earth content of this phosphor blend is over 40% by total weight of the phosphor composition. Because rare earth phosphors can be expensive to use, attempts have been made to achieve high CRI and yet reduce the rare earth phosphor content so that the lamp can be made more economically. Such attempts have been successful only in lamps that exhibit a very high CCT, in excess of 5000K. Thus, a trade-off must be made between reducing the cost of the phosphor and maintaining a low CCT.
Therefore, a need exists for a phosphor blend that provides CRI of at least 87 or better, while at the same time achieving low CCT, below about 4500K, and that is more economically feasible. The use of a phosphor blend including only up to about 20% rare earth phosphor in combination with certain non-rare earth broad band phosphors in accord herewith provides a lighting solution having improved CRI while maintaining a low operating CCT, that can be manufactured at lower cost due to the reduction in the amount of rare earth phosphor included in the phosphor blend.